The scattering properties of tissue are such that amplitude and instantaneous frequency deviations are induced in the backscattered echo signals. The instantaneous frequency of the ultrasound signal is defined as the time rate of change of the signal's phase. Most conventional ultrasound systems generate images based upon amplitude fluctuations in these ultrasonic waves reflected from the tissue. While the amplitude varations induced by the tissue are useful for determining the boundaries of various body organs and also provide some textural information, instantaneous frequency variations induced by the tissue lead to frequency demodulated images that may be more useful than the amplitude demodulated images in characterizing the textural information.
Nearly all conventional ultrasound systems have a block diagram which is similar conceptually to that shown in FIG. 1. Such systems are referred to as "B-scanners," echo scanners or real time ultrasound scanners. These scanners project an acoustic pressure signal, in the form of an ultrasonic pulse, against the object, and measure the acoustic pressure signal resulting from the scattering of the ultrasonic pulse along its path of propagation--i.e., the ultrasonic "echo" signals. These ultrasonic echo signals are referred to as "A-lines."
Referring to FIG. 1, the ultrasonic pulse is usually generated by a transmitter and transformed into an acoustic pressure signal by a transducer 10 mounted upon an arm 20 moved relative to the object to project the acoustic pressure signal against various parts of the organ. The transducer 10 is also connected to a receiver which measures the ultrasonic echoes. The output of the receiver is connected to an envelope detector, which is an amplitude demodulator. The envelope detector removes the high frequency component of the reflected signal. The output of the envelope detector is connected to conventional circuit means 30 to generate a video signal containing acoustic images of the specimen. The output circuit 30 is coupled to a digital scan converter. The scan converter is also coupled to the arm 20 upon which the transducer 10 is mounted, in order to receive positional information. Digital scan converters, as known in the art, algorithmically process video signals by a predefined algorithm to generate an image of the entire specimen. The output of the digital scan converter is then displayed on a conventional cathode ray display.
Careful analysis of the signal received by conventional B-scanners reveals a substantial amount of instantaneous frequency deviation in the RF "carrier" portion of the signal, i.e., the high frequency components. Instantaneous deviations of plus or minus 500 Khz on a center frequency of 3 Mhz have been measured. However, the prior art does not teach any useful means of using this information.
Accordingly, it is the principal object of the present invention to use the instantaneous frequency deviations of the backscattered signal which are dependent on the scattering properties of media supporting acoustic pulse propagation to generate acoustical images of the media.
A further object of the present invention is to generate acoustic images of media utilizing the instantaneous frequency deviations of signals derived from reflected ultrasonic waves.
Yet another object of the present invention is to generate color ultrasonic images based on the deviations in amplitude and frequency in backscattered ultrasonic signals.